This invention relates to an aqueous alkaline cell with a cathode comprising copper oxide or copper hydroxide, particularly such cathodes with a conductive additive of expanded graphite or graphitic carbon nanofibers and sulfur.
Conventional alkaline electrochemical cells have an anode comprising zinc and a cathode comprising manganese dioxide. The cell is typically formed of a cylindrical casing. The casing is initially formed with an enlarged open end and opposing closed end. After the cell contents are supplied, an end cap with insulating plug is inserted into the open end. The cell is closed by crimping the casing edge over an edge of the insulating plug and radially compressing the casing around the insulating plug to provide a tight seal. A portion of the cell casing at the closed end forms the positive terminal.
Primary alkaline electrochemical cells typically include a zinc anode active material, an alkaline electrolyte, a manganese dioxide cathode active material, and an electrolyte permeable separator film, typically of cellulose or cellulosic and polyvinylalcohol fibers. The anode active material can include for example, zinc particles admixed with conventional gelling agents, such as sodium carboxymethyl cellulose or the sodium salt of an acrylic acid copolymer, and an electrolyte. The gelling agent serves to suspend the zinc particles and to maintain them in contact with one another. Typically, a conductive metal nail inserted into the anode active material serves as the anode current collector, which is electrically connected to the negative terminal end cap. The electrolyte can be an aqueous solution of an alkali metal hydroxide for example, potassium hydroxide, sodium hydroxide or lithium hydroxide. The cathode typically includes particulate manganese dioxide as the electrochemically active material admixed with an electrically conductive additive, typically graphite material, to enhance electrical conductivity. Optionally, small amount of polymeric binders, for example polyethylene binder and other additives, such as titanium-containing compounds can be added to the cathode.
The manganese dioxide used in the cathode is preferably electrolytic manganese dioxide (EMD) which is made by direct electrolysis of a bath of manganese sulfate and sulfuric acid. The EMD is desirable since it has a high density and high purity. The electrical conductivity (resistivity) of EMD is fairly low. An electrically conductive material is added to the cathode mixture to improve the electric conductivity between individual manganese dioxide particles. Such electrically conductive additive also improves electric conductivity between the manganese dioxide particles and the cell housing, which also serves as cathode current collector. Suitable electrically conductive additives can include, for example, conductive carbon powders, such as carbon blacks, including acetylene blacks, flaky crystalline natural graphite, flaky crystalline synthetic graphite, including expanded or exfoliated graphite. The resistivity of graphites such as flaky natural or expanded graphites can typically be between about 3xc3x9710xe2x88x923 ohm-cm and 4xc3x9710xe2x88x923 ohm-cm.
It is desirable for a primary alkaline battery to have a high discharge capacity (i.e., long service life). Since commercial cell sizes have been fixed, it is known that the useful service life of a cell can be enhanced by packing greater amounts of the electrode active materials into the cell. However, such approach has practical limitations such as, for example, if the electrode active material is packed too densely in the cell, the rates of electrochemical reactions during cell discharge can be reduced, in turn reducing service life. Other deleterious effects such as cell polarization can occur as well. Polarization limits the mobility of ions within both the electrolyte and the electrodes, which in turn degrades cell performance and service life. Although the amount of active material included in the cathode typically can be increased by decreasing the amount of non-electrochemically active materials such as polymeric binder or conductive additive, a sufficient quantity of conductive additive must be maintained to ensure an adequate level of bulk conductivity in the cathode. Thus, the total active cathode material is effectively limited by the amount of conductive additive required to provide an adequate level of conductivity.
Although such alkaline cells are in widespread commercial use there is a need to improve the cell or develop a new type of cell that is cost effective and exhibits reliable performance as well as high capacity (mAmp-hours) and high service life for normal applications such as flashlight, radio, and portable CD players.
An aspect of the invention is directed to a primary (nonrechargeable) electrochemical alkaline cell having an anode comprising zinc and a cathode mixture comprising copper oxide (CuO) or copper hydroxide (Cu(OH)2) cathode active material. If copper oxide is employed in the cathode mixture, its purity is desirably at least 97, preferably at least 99 percent by weight, desirably between about 97 and 99.8 percent by weight, e.g. about 99.5 percent by weight. A graphitic carbon is added to the cathode mixture. It has been determined that a graphitic carbon comprising expanded graphite or graphitic carbon nanofibers or mixtures thereof provides a very suitable conductive additive. Such graphitic carbon material significantly reduces the cathode resistance, elevates the cell""s running voltage and increases cell capacity and performance. The addition of expanded graphite or graphitic carbon nanofibers to cathodes comprising copper oxide or copper hydroxide is particularly desirable. The conductive material thus is desirably composed essentially entirely of expanded graphite or graphitic carbon nanofibers or mixtures thereof. Preferably, the cathode mixture comprises between about 3 and 10 percent by weight, preferably between 4 and 10 percent by weight of the graphitic carbon nanofibers. Preferably, the graphitic carbon nanofibers have a mean average diameter less than 500 nanometer, more preferably less than 300 nanometers. Desirably the graphitic carbon nanofibers have a mean average diameter between about 50 and 300 nanometers, typically between about 50 and 250 nanometers.
The anode and cathode include an aqueous alkaline solution, preferably aqueous KOH solution. The cathode desirably comprises between about 4 and 10 percent by weight of the conductive additive. The copper oxide is preferably in the form of a powder having an average particle size between about 1 and 100 micron. The cathode mixture includes an aqueous KOH solution, desirably having a concentration of between about 30 and 40 percent by weight, preferably between 35 and 45 percent weight KOH in water. The aqueous KOH solution desirably comprises between about 5 and 10 percent by weight of the cathode mixture. The cathode active material comprising copper oxide, preferably comprises between about 70 and 92 percent by weight of the cathode mixture.
In an aspect of the invention the cathode can comprise copper hydroxide. In such case the cathode desirably comprises between 65 and 90 percent by weight of the copper hydroxide (calculated on the basis of pure copper hydroxide). The cathode can also comprise a mixture of copper oxide and copper hydroxide in which case said copper oxide and copper hydroxide mixture preferably comprises between about 65 and 92 percent by weight of the cathode.
In an aspect of the invention wherein the cathode mixture of the primary alkaline cell comprises copper hydroxide as cathode active material, the cathode mixture preferably also includes sulfur. The copper hydroxide is desirably added to the cathode in the form of a copper hydroxide rich additive comprising at least 90 percent by weight pure copper hydroxide, since it is expensive to obtain copper hydroxide in 100% pure form. The sulfur is elemental sulfur in the thermodynamically favored state, known as the orthorhombic or xcex1 form. The sulfur can also exist in any of its natural forms such as polymeric, xcex2 or xcex3 sulfur. A suitable sulfur is also available under the trademark Crystex, which is a rubber insoluble sulfur. The orthorhombic form of sulfur is preferred. The elements selenium (Se) or tellurium (Te) and mixtures thereof can be employed in place of or in admixture with the sulfur additive. The sulfur elevates the running voltage of the cell and enhances the cell performance, which in turn leads to increased power and cell life. The performance of the cell is further enhanced when the cathode also comprises a graphitic carbon comprising expanded graphite or graphitic carbon nanofiber in addition to the sulfur. However, the graphitic carbon can also be natural flaky crystalline graphite. The sulfur desirably comprises between about 1 and 15 percent by weight of the cathode, preferably between about 5 and 10 percent by weight of the cathode.
The cathode desirably comprises copper hydroxide additive in amount between about 65 and 90 percent by weight of the cathode (calculated on the basis of pure copper hydroxide), desirably between about 85 and 90 percent by weight of the cathode. The copper hydroxide additive desirably has a range of particle sizes between about 1 and 100 micron), and a mean average particle size between about 15 and 25 micron). The copper hydroxide can desirably be supplied in the form of a copper hydroxide additive which has a purity of between about 90 percent by weight and 99 percent copper hydroxide by weight or higher (even up to 100 percent purity), desirably a purity between about 92 and 99 percent by weight or higher. Preferably, the copper hydroxide additive in the cathode has a purity of between about 94 and 99 percent by weight or higher copper hydroxide. Metal impurities which include iron and chromium metals in elemental or combined form are desirably removed from the copper hydroxide additive so that the cathode has less than 100 ppm, preferably less than 15 ppm total amount of such metal impurities therein. When copper hydroxide is employed as cathode active material, the graphitic carbon, preferably expanded graphite, or graphitic carbon nanofibers and mixtures thereof desirably comprises between about 3 and 10 percent by weight of the cathode, preferably between about 4 and 8 percent by weight of the cathode. The cathode mixture includes an aqueous KOH solution, desirably having a concentration of between about 25 and 40 percent by weight, preferably between 30 and 35 percent weight KOH in water. The aqueous KOH solution desirably comprises between about 5 and 10 percent by weight of the cathode mixture. The cell preferably comprises no added mercury, that is, less than 50 parts by weight mercury per million parts total cell weight. The cell preferably does not contain added amounts of lead and thus can be essentially lead free, that is, the total lead content is less than 30 ppm (parts per million), desirably less than 15 ppm of total metal content of the anode.
It is not known with certainty why voltage and cell performance, e.g. increased energy output, improves with the addition of sulfur to alkaline cell cathodes comprising copper hydroxide as active material. The addition of sulfur may be reacting with the copper hydroxide to form intermediate sulfur compounds which can improve the rate at which electrochemical reaction occurs during cell discharge. This can reduce the cell""s internal resistance and elevate the running voltage. A portion of the sulfur or intermediate compound can also be participating in the electrochemical reaction as well. The addition of sulfur and graphitic carbon comprising natural graphite, expanded graphite or graphitic carbon nanofibers or mixtures thereof to alkaline cell cathodes comprising copper hydroxide, appears to have a synergistic effect in improving cell performance. The use of expanded graphite in admixture with sulfur for alkaline cell cathodes comprising copper hydroxide appears to improve cell performance the best. This may be due in part to the high surface area and geometrical configuration of the expanded graphite.